System for tracing telephone calls



April 20, 1937. .1. w. TAYLOR 2,077,537

SYSTEM FOR TRACING TELEPHONE CALLS Filed May 19, 1936 '2 Sheets-Sheet l S P gw! Y WIV" l` VH MHH. M u E NQ m QN\/ I Q E Euwe [QQ W L .w h QS .B #S VSS /Nf/EA/TOR J. n. TA VL 0R e y ,4 Tram/,5y

April 20, 1937. J. W. TAYLOR SYSTEM FOR TRACING TELEPHONE CALLS Filed May 19, 1936 2 Shee'ts-Sheet 2 GNN w mw /A/L/E/v TOR J. W. TA VL 0R "l called party. "However,

Patented Apr. 20, 1937 PATENT -ozFFlcE f SYSTEM: FOR 'IRACING TELEPHONE CALLS -John'W..jTaylor, Omaha, Nebr., assignor vto AmericanzTelephone andTelegraph Company, a corporation of' New York :iApplicationfMay 19, 1936, Serial No. 80,614

' 12 Claims.

-This application relatesitoy `telephone systems, and more `particularlyitofcircuitsforenabling the-origin of calls. in dial switchingexchangesto be traced.

5 Subscribers are sometimes seriously annoyed by vcalls which are repeatedly :madeitov their :lines by personswhosesole objectwis fto maliciously annoy them. At-other times'awhen .callsf are made to a particularline for the purposezof makl0 ing blackmailing vor extortional requests, it is highly desirable to'- be able to' trace the origin of such calls. In manualfexchange` areasgit isa comparatively 'simple matterl for anvv operator .to trace the'origin of anyca'll at the requestxof' the in exchange areas equipped with dial-y switchingf equipment not requiring the services of an operator to establish connections, it `is-very diicult to trace=-such .annoyance calls since-an established connection is 20 interrupted as -soon as 'the calling party l'restores his receiver -fto the switchhook.

-It is therefore the -"objectof theinvention to provide facilities applicable to av -di'alI switching lexchangearea to-'enable any subscriber whovhas 25 complained-of being annoyed by such malicious callsto control-'the tracing of Lsuch calls whereby Aacentraloflice switchboard attendantmay quickvly and easilyv ascertain the origin of them.

Inraccordancewith the present invention, 30=when a subscriber entersa' complaint that'he is being annoyed, a-high frequency oscillator'istemporarily associated withV his line'terminals; which isarrangedlto be-bridged across his'iline'at the central oiceunder 'the control of ar'keyinstalled Slat' the subscribers premises,for transmitting a "high frequency tone`^back over the `connection built up to-his`-1ine from the calling line causing yfthe* annoyance. A iinder circuit comprising a plurality of hunting-switches connected in rpy- 40 ramid formation is alsoA providediat fthe central fce which is -startedinv response tov the operation ofthe key at the calledv subscribers premises and successively tests for lthe .tone at all selector w switches of yone of theswitching stages ofthe u cnice. onofces. of .the .exchange area.

`Vihenlthelparticular.selector switch over which the annoyance-call has beenmadeand vonwhich thehigh vfrequency toneis present .has lbeen found,

50 such-switch` is held by the finder circuit so that the-callingl subscriber maynotvrelease the Sconnectiongtheinder circuit'isloclred` in its setpcsition and an alarm signal is operated. A switch- VVVroom attendant zhearing-fthei alarm signalifmay {v51-then readily' :tracey the :call since theposition assumedby thegnder circuit will identify tov-him the particular selector switch 'of the rstl switch- Ving stage over'which the call has been madeand the connection ymay then be ,traced therefrom over the line-finder or line` switchd-irectly ytothe terminals of the calling line.

For a clearer understanding-'of theinvention, reference mayl be had' to: Vthe following ldetailed description thereof and the manner in .which it functions, taken in connectionwith -theaccompanying drawings, in which Figure lshows schematically a'calling. line,a called line Yto which repeatedA annoyance callsare assumed, to; halvecbeenl made, selector switches overwhich aconnectionhas'been established-ibetween such lines; an oscillator circuit; for; generating a high frequency tonecurrent; and tone; deftection andalarm circuitsyand Fig'. 2 shows a in'der circuit for,-succesisiyely associating the tone detection circuit` of Fig.z 1 with all district selector circuitslof the exchange, vone such district selector circuit. being disclosed in Fig. 1.

For thev purposes, of illustrationvthe, invention has been disclosed asapplied .toa'dial switching telephone exchange .system of thenwellr-known v panel =type,-although :it is to beunderstoodjthat the invention is equally applicable toialll types of; dial switching systems.

The oscillator ltl'ldsclosed in Fig. 1 mayzbev of `the. well-known vacuum tube type designedrto generate a tone currentof 630.00 cyclesv frequency. VAny other well-known;` type kof highftrequency currentlgenerator mightA be employed capable `of generating tone current ofanyldesired'frequency. -Tone current detector 125 is also of-'the vacuum tube type and is designed to be responsive only `to the high frequency generated by the oscillator II-50.

In a small exchange area having few oiices, one finder circuit such as is disclosed inf lig.v 2, together with a detector circuit 125 disclosed in JFig.' lywould beprovided in eachofce. =This nder circuit comprises a frame finder or'fmaster switch 200 and a maximum of fourteen secondary district selectors or all the selectors of one frame,

as many secondary switches are provided as there are frames of district selectors. In small exchange areas having few ofces, a maximum of fourteen secondary nder switches and one associated frame nder switch is adequate for each office. Each secondary nder switch is provided with three arcs of test terminals with cooperating test brushes in which arcs test conductors extending to sixty district selectors terminate, and is arranged during three half revolutions to successively establish testing circuits to the sixty district selectors. For example, secondary finder switch 2I0, which has access to sixty district selectors on frame No. 1, will establish testing circuits over brush 2| I to twenty district selectors on that frame during its rst half revolution, to twenty other districtv selectors over brush 2 I2 during its second half revolution and to twenty further selectors over brush 2|3 during its third half revolution. The frame finder switch 200 during its step-by-step advancesuccessively renders the several secondary finder switches effective, causes the advance of each `secondary switch through three half revolutions and successively renders the three test brushes of each secondary nder effective.

In larger exchange areas where more than fourteen frames of district selectors may be required in each office, a second finder circuit, such as is disclosed in'Fig. 2, together with an associated detector circuit |25, would be required in each oce controlled by a second start relay I I3' connected in parallel with start relay I|3.

It will be assumed that the subscriber whose substation |00 is disclosed in the right portion of Fig. l has entered a complaint that he is being annoyed. `For the purpose of tracing the origin of such annoyance calls the output circuit of the 'oscillator |50 is associable by conductors IOI and |02 and a connection shoe |03 with the tip and ring conductors of the subscribers line on the line side of the distributing frame, and the circuit of relay |04 is extended over a spare conductor |05 of the cable in which the subscribers line is located to` the subscribers premises where it terminates in the contacts of a key |06 placed in proximity to the subscribers substation telephone set.

It will also be assumed that thereafter an annoyance call is made from the substation |01 and is completed to the called subscribers line in the usual manner over the line-finder |08, district selector |09, office selector I I0, incoming selector III and nal selector II2. The called subscriber upon recognizing the call as of the annoyance type will operate the key |05 and keep the calling party in conversation. Upon the operation of key |06 the circuit of relay |04 is completed over conductor |05 thereby closing the start circuit of the oscillator |50 over its outer right contacts, connecting the output circuit of the oscillator over its left contacts and conductors IOI and |02 to the tip and ring terminals of the called line and closing a start circuit for start relay I I 3 of the nder circuit of Fig. 2 which will be assumed to be in the office in which the call originated and, if required, in parallel to the winding of relay IIS of a second iinder circuit in the originating office similar to Fig. 2 and also in parallel to the windings of similar start 4relays of similar nder circuits located in `the other oflces of the exchange. VThe circuit of relay II3 may be traced from ground over the inner right contact of relay |04 to battery through the Winding of relay II3. 4

Relay I I3 upon operating connects ground over its front contact to conductor I I 4 to light the filament of the detector tube IIE, to operate dash-pot relay |31 and to supply operating and locking ground for the relay equipment of the detector circuit |25. Relay |31 is made slow to operate to allow suicient time for the filaments of the tubes of oscillator |50 and detector tube II5 to become heated and when fully operated establishes an operating circuit for the stepping magnet 201 of the frame iinder switch 200. This circuit may be traced from ground through the interrupter 208, winding of magnet 201, brush 205 of frame nder switch 200 and the iirst terminal of its associated terminal arc, conductor II1, contacts of relay I 31, conductor IIE, brush ZIB and normal terminal of nder switch 2I0 to battery through the winding of stepping magnet 2|1 of iinder switch 2I0.

As a result of the closure and opening of this circuit by the .interrupter 208, both switches 200 and 2 I0 advance one step whereupon the stepping circuit is opened by the movement of brush 2|6 from its normal bank terminal and switch 200 is arrested with its brushes in engagement with the next terminals of their respective arcs. However, when switch 2I0 has advanced its brushes one step, an intermittent stepping circuit is established for switch 2I0 which may be traced from battery through the winding and interrupter contacts of stepping magnet 2|1, through resistance 2 I8, the ofi-normal strapped terminals of the arc with which brush 2 I 5 cooperates, brush 2|5 to ground, and switch 2I0 continues to ad- Vance through a complete half revolution, or until the brush 2|5 leaves the last terminal of its arc.

As the brush 2II successively engages the terminals of its arc, test circuits are established from the ring conductors of the rst twenty district selectors of frame No. 1, over brush 2| I, the second terminal of the arc upon which brush 20| of frame finder switch 200 is now positioned, brush 20|, conductor II8, condenser |20 and the primary winding of transformer I 2| to ground, but without effect since it will be assumed that the calling line |01 has established a connection to the called line |00, over a district selector switch on frame No. 2 and the high frequency current from oscillator I 50 is present only on district selector No. 17 of frame No. 2.

When the brush 2I6 again engages the normal terminal of its arc after completing a, first half revolution, a stepping circuit is established from interrupter 208 through the winding of stepping magnet 201 and the second terminal of the arc upon which brush 205 of frame nder switch 200 is now resting, brush 2 I6 and the normal terminal of its arc, winding of stepping magnet 2|1 to battery. Switch 200 is now advanced another step and finder switch 2I0 is advanced one step off normal, opening the stepping circuit just traced and arresting the further advance of switch 200. Finder switch 2I0, however, continues to advance through a second half revolution under the control of the local stepping circuit established over brush 2|5, the brush 2I2 during this half revolution extending test circuits from the second twenty district selectors on frame No. 1 over the third terminal of the arc with which brush 20| of switch 200 cooperates, thence as traced through the primary winding of transformer I2| but without effect.

As soon as the nder switch 2I0 completes its second half revolution, the local stepping circuit `phrases' through magnet 2|1 is opened at brush- 2|5r and the previously traced stepping circuit -from'interrupter 4208 is again completed throughjthewinding of `stepping-magnet 201 and the :thirdterminal of the arcfupon which brush 20501? switch 200. is now resting,brush 2|6-and the normal terminal of its arc and thewindinggcf steppingmagnet 2I1 tobattery. -Switch 200 =isnowadvanced another step and iinder switch 2| 0 is advanced one step ofi normal, opening the stepping circuit just traced and arresting the further advance of switch i 200. Finder switch 2|0, however, continues to advance through a thirdhalf-revolution under the control of its stepping circuit established over brush 2I5, `the brush 2.I3 during this half revolution extending test circuits from the third twenty district selectors on frame'No. 1 over the fourth terminal-of the arc with'which brush 20| of switchf200 cooperates, thence as traced through the primarywinding of transformer I2I but without effect.

When finder switch 210 completes-its third half revolution, the local stepping` circuit through magnet 2I1 is opened at brush 2| 5 and a stepping circuit is established from interrupter-Z'through the winding of step-ping magnet .201 and the fourth terminal of thearc upon which brush 205 of switch 200 is now resting, the normal terminal of the arc of finder switch 210 upon which brush 2I4 is now resting, the normal terminal of the arc of finder switch 220 upon which brush 226 is now resting, winding of stepping magnet 221 to battery. Switch 200 is now, advancedv another step and finder switch 220is` advanced one step off normal, opening the 'stepping circuit just traced and arresting the further advance of switch 200. rFinder switch 220, however, continues to advance through a first half revolution over a local circuit extendingfrorn battery, winding and interrupter contacts ofmagnet 221, resistance 228, strapped terminals andibrush 225 to ground,.successively completing test circuits from the first twenty district selectors on frame No. 2 over the terminals cf its `upper arcand brush 22 I, the fth terminalof the arc with which ybrush 20| of switch 200 cooperates, thence as v oscillator.

traced through the primary winding of transformer |2|.

It, was assumed'that the No. 1'1 district selector of frame No. 2 was used in the establishment of the annoyance'call' to the subscribers substation y|00 and that, therefore, the high frequency tone from oscillator I50-is present at this district-selector, flowing from the oscillator l over the inner left contact of -relay |04, conductor 40|, ring brushes of selectors |I2, II I, I I0 and |09, right windings of lrepeating coil |22 at the district selector '|09, returningover the tip brushesof selectors 1|99, ||0,` |IIv and H2, conductor |02, outer left contacts-of relay |04 to the This tonecurrent is therefore induced in a circuit `which :is established' when the test brush 22| of nder switch 220 engages its seventeenth terminal 229 which may be traced as follows: Grounded batteryflowerleftwinding of repeating coil |22-at1the district selector |09, winding of supervisory. lrelay |l23,1conductor |24, terminal 229,.brush 22|, ythe fifth-terminal upon which brush 20| of switch 20(1.is` resting, thence as traced to ground through the primary winding of transformer AI 2 I.

This high frequency tone current vis now induced upon thesecondary winding of transformer Y I2I and upon the gridfof thedetectontube H5.

The grid of tube I I5 is'normally negativelybiased by .theabattery |-26sand Vblocks the vflowfof space vcurrentfthrough the tube. When,Vv however, the

`high .frequency tone current from the. oscillator ',I50nisfimpressed on the gridof the-tube through i the band filter comprising :impedance |28 and condenser |29, which is tunedwto pass only the particularfrequencygenerated by the oscillator across. the rectifier |32.

Relay |21 upon operating closes a circuit for yrelay V|24 extending, fromground on conductor -Iildcver its upper` contact-'winding ofv relay |34 .rto'fbattery thereby operating relay |34 which .flocks `over'its lower front-contact to ground on -iconductor I|.4, independentlyof relay |21. Re-

lay |21 also establishes a circuitfrom ground on conductor' II4 over its inner upper front contactfconductor |36, brush 203 of master switch 1.200; and the iifth terminal of the arc-upon which i it is resting; winding of stepping` magnet 221 to *'hattery; thereby holding vmagnetf221 energized -and preventing the further advance of the brushes of ifnder switch 220. Since the operation of Vtube H5 and the consequent operaticnof relay |21.haszoccurred immediately upon the engagement of'brushA 22| with terminal 229,;the brush- .es of; finderswitchr 220 arestopped with the brush: 22 I` .restingi upon terminal: 229.

VRelay .I 24 uponV operating closes an obvious cir- ;icuitfiforthe .alarmdeviceil`35 and connects ground .through impedance II9 to conductor |I8. The connection of ground to conductor I I8 establishes a holding circuitfor the supervisory relay |23 s of the district selector |09 whichA may be traced from ground on conductor |I8, brushes 20| and 22|, conductor |24, winding of relay |23, lower left winding of repeating coil |22, togrounded battery, thus preventing the release of relay |23 and the release of the established connection should the calling subscriber restorehis receiver tothe switchhook.

The switchboard attendant at the oflice in which the call originated hearing `the'alarm device |35 is apprised of the factthat an annoyance call has been originated and detected. Hemay r then readily trace the call by iirst noting theposition in whichthe frame finder of' switch 200` is standing, thereby'indicating the particular secondary finder switch lockedin an off-normal position and as a consequence Athe particular frame of district selectors in which the district selector employed in the connection is located. The attendant may thenfnote the position assumed by the -secondarypnder switch, in the case assumed switch- 220, and thereupon ascertain that eitherthe 17th, 37th or 57th district selector on the No.

.2.,district frame is involvedwin the connection. .For this purpose the master switch y200 and the nder switches may be equipped with indicators,

not shown, provided withnumbers correspondi ing to `:the'district selector frame andthe identiiicationof the district selectors on 'fea-ch frame.

Having located thedistrict selector as onenof three on the No. 2.: district frame and since all selectors; employed inxfthe establishment-:of the connection have been locked against release, the attendant may by an inspection of these three district selectors readily determine the line-finders associated therewith and trace the connection to the terminals of the calling line at the bank of the line-finder |08.

Having traced the origin of the connection, the attendant may communicate with the called subscriber in any desired manner and the called subscriber may then proceed to release the linder circuit by releasing key |06, thereby releasing relay |04 to disconnect the oscillator |50 from the terminals of his line and releasing relay ||3, Rel-ay ||3 upon releasing removes ground from conductor ||4, thereby opening the filament circuit of detector tube ||5 and releasing relay |34. Relay |34 upon releasing opens the circuit of alarm device |35 and opens the previously traced circuit over conductor IIB, brushes and 22| of finder switches 200 and 220 and conductor |24 by which the district selector |09 was held from releasing. As soon as both the calling and called subscribers return their receivers to their switchhooks the established connection over selector switches |08, |09, ||0, and ||2 becomes released in the usual manner.

When the oscillator |50 became disconnected from the called line through the release key |06, the detector tube ||5 was rendered ineffective to maintain relay |21 operated and relay |21 released. With ground removed from conductor ||4 and relay |21 released, the circuit previously traced over conductor |36 and brush 203 of frame nder switch 200 by means of which the district nder 220 was held in its operated position is opened and nder switch 220 is now advanced to complete its rst half revolution by a self-interrupting circuit extending from battery through the winding and interrupter contacts of magnet 221, resistance 220, over the strapped terminals of the arc associated with brush 225 and brush 225 to ground.

As soon as the finder switch 220 completes its half revolution the local stepping circuit through magnet 221 is opened at brush 225 and a stepping circuit from interrupter 208 is again completed through the winding of stepping magnet 201 and the iifth terminal of the arc upon which brush 205 is now resting, brush 2M and its normal terminal, brush 226 and its normal terminal, winding of magnet 221 to battery. Switch 200 is now advanced another step and finder switch 226 is advanced one step oiT-normal, opening the stepping circuit just traced and arresting the further advance of switch 200. Switch 220, however, continues to advance through a second half revolution under the control of its local stepping circuit. When the iinder switch 220 completes its second half revolution, the local stepping circuit is opened at brush 225 and a stepping circuit is established from interrupter 208'through the winding of stepping magnet 261 and the sixth terminal of the arc upon which brush 205 is now resting, the normal terminal of the arc of finder switch 2|0 upon which brush 2M is now restin-g, winding ci stepping magnet 221 to battery. Switch 200 is now advanced another step and nder switch 220 is advanced one step oi normal, opening the stepping circuit just traced and arresting the further advance of switch 200. Finder switch 220, however, advance-s through a third half revolution by reason of its local stepping circuit until brush 225 reaches its normal position.

With switch 220 in its normal position and switch 200l in position 1, a circuit is now established from interrupter 208 through stepping magnet 201 over brush 205 and the seventh tervminalof its arc, brush 224 of nder switch 220 and its normal terminal, and thence over the normal terminal and brush of the next finder, corresponding to brush 226, and the stepping magnet of such nder to battery, whereupon the frame finder 200 is advanced one step and the next district nder is moved one step olf normal. In the manner previously described, each of the remaining district iinders including nder 230 is advanced through three half revolutions and the frame iinder is advanced step by step. It will be noted in this connection that when the district frame nder is standing with its brush 205 in position 22, the district finder, not shown, serving the No. '1 district frame will be started on its rst half revolution and the frame finder 200 will advance its wipers one step, thereby positioning brush 206 on the rst terminal of its arc. The advance of the No. '1 district iinder to complete its second and third half revolutions will now be controlled over brush 206 of the district frame finder 200 and the advance of subsequent district nders including finder 230 will also be controlled over brush 206 of district frame nder '200.

When the last district nder 230 is advanced 01T normal to start its third and last half revolution, the frame nder 200 is advanced one step to position wiper 206 on the twenty-iirst terminal of its arc, whereupon a. circuit is established from the interrupter 208, winding of stepping magnet 201, brush 206 and the twenty-rst and twenty-second terminals of its arc strapped together, conductor 209, back contact of relay ||3, now released, resistance |38 to battery. Switch 200 is now advanced two steps, whereupon the stepping circuit just traced is opened at brush 206 and brushes 20|, 203 and 205 are advanced into engagement with the lirst position terminals of their arcs. All of the apparatus of the finder circuit is now in normal condition preparatory to enabling the tracing of a subsequent call.

In office areas wherein certain oiiices may be located at such distances from other offices as to render diiicult the transmission of the tone current over the interoilice trunks in suihcient volume for detection in the originating office, iinder and detector circuits of the character disclosed could then be installed in the office in which the line of the annoyed called subscriber terminates and the nder circuit would then have access to the incoming selector switches of the terminating cnice. Upon the testing of an incoming selector switch on which the tone current was found to be present, the nder switch could then apply another source of tone to the interofce trunk and start the finder circuit of the origihating office to Search for the district selector of the originating oiiice to which such second source of tone current was transmitted over the interoice trunk.

What is claimed is:

l. In a system for tracing the origin of telephone calls, a plurality of switching stages over which calls may be established from calling lines to called lines, each stage comprising a plurality of selector switches, a source of tone current, a tone current detector, a -finder circuit for associating said detector in rotation with each selector switch of one of said switching stages, and means controlled from a called line for connecting said source of tone current tothe central/cnice terminals-fof said-called line and for causing said nder circuit to search forthe selectorswitch of said one switching stage over which a call has been established to said called line.

2. In a system for tracing the origin oftelephone calls, a pluralityl of switching stages over which calls may be established from calling lines to called lines, each stage comprising aplurality of selector switches, a source of tone current, a tone current detector, a finder circuit for associating said detector in rotation with each selector switch of the i'lrst switching stage, andmeans controlled from a called line for connecting said surce of tone current to the central office terminals of said called line and forcausing said iinder circuit to search for the selector switch of the rst switching stage over which a callr has been established to said called line.

3. In a system for tracing the origin of telephone calls, a plurality of switching stages over which calls may be established from calling lines to called lines, each stage comprising a plurality of selector switches, a source of tone current, a tone current detector, a finder circuit for associating said detector in rotation with each selector switch of one of said switching stages, means controlled from a called line for connecting said source of tone current to the central oilice terminals of said called line and for causing said iinder circuit to search for the selector switch of said one switching stage over which a call has been established to said called line, and an alarm device operated by said detector when said particular selector switch has been found.

4. In a system for tracing the origin of telephone calls, a plurality of switching stages over which calls may be established from calling lines to called lines, each stage comprising a plurality of selector switches, a source of tone current, a tone current detector, a nder circuit for associating said detector in rotation with each selector switch of one of said switching stages, means controlled from a called line for connecting said source of tone current to the central oilice terminals of said called line and for causing said finder circuit to search for the selector switch of said one switching stage over which a call has been established to said called line, and means controlled by said detector for arresting the hunting movement of said nder circuit in the position to which it has advanced when the selector switch of said one switching stage on which the tone current is present has been selected.

5. In a system for tracing the origin of telephone calls, a plurality of switching stages over which calls may be established from calling lines to called lines, each stage comprising a plurality of selector switches, a source of tone current, a tone current detector, a finder circuit for associating said detector in rotation with each selector switch of one of said switching stages, means controlled from a called line for connecting said source of tone current to the central oflice terminals of said called line and for causing said nder circuit to search for the selector switch of said one switching stage over which a call has been established to said called line, and means controlled by said detector for preventing the calling subscriber from releasing the connection established to said called line.

6. In a system for tracing the origin of telephone calls, a plurality of switching stages over which calls may be established from calling lines to called lines, each stage comprising a plurality of selector switches, a source of vtone current, a

tone current detector, a rlndercircuit'for associating said detector in rotation with each selector switch of .the first switching stage, means controlledfrom a called line for connecting said source of 'tone current to the central oiiice terminals of said called line and for causing said finder circuit to search for the selector switch of the first switching stage over which a call has been established to said called line, and means controlled by said detector for preventing 'the calling subscriber from releasing the connection established to said called line.

7. In a system for tracing the origin of telephone calls, a plurality of switching stages over which calls may be established from calling lines to called lines, each stage comprising a plurality of selector switches, a supervisory relay in each switch of said rst stage'controllable by a calling subscriber to govern themaintenance of a connection established from the calling line, a source of tone current, a tone current detector, a iinder circuit for associating said detector in rotation with each selector switch of said rst switching stage, means controlled from a called line for connecting said source of tone current to the central oice terminals of said called line and for causing said finder circuit to search for the selector switch of said irst Switching stage over which a call has been established to said called line, and means controlled by said detector over said finder circuit for locking the supervisory relay of said selector switch to prevent the calling subscriber from releasing the connection established to said called line.

8. In a finder circuit for selecting any one of a plurality of circuits which are arranged in main groups divided into a plurality of subgroups, a plurality of nder switches each individual to a main group of said circuits and having access to the subgroups of said main group of circuits over different testing brushes and associated arcs of terminals, a master switch for selecting said finder switches in rotation, and means controlled by said master switch for rendering the brushes of each selected nder switch successively effective to test the subgroups of circuits to which they have access.

9. In a finder circuit for selecting any one of a plurality of circuits which are arranged in main groups each divided into a plurality of subgroups, a plurality of finder switches each individual to a main group of said circuits and having access to the subgroups of said main group of circuits over different testing brushes and associated arcs of terminals, a master switch for selecting said nder switches in rotation, and means controlled by said master switch for causing ,a plurality of revolutions of each finder switch following its selection and for rendering the brushes of that finder switch successively effective upon successive revolutions to test the subgroups of circuits to which they have access.

10. In a finder circuit for testing any one of a plurality of circuits which are divided into a plurality of subgroups, a nder switch operable through a plurality of cycles and having access to said subgroups, a master switch, a stepping magnet for each of said switches, an operating circuit for said magnets eiective upon each closure thereof to advance said master switch one step and to cause said finder switch to advance through one complete cycle, and means controlled by said master switch in its successive `positions for rendering said finder switch eiective to test a different subgroup of said circuits during each operating cycle thereof. l Y 11. In a finder circuit for testing any one of 5 a plurality of circuits which are divided into a plurality of subgroups, a finder switch operable through a plurality of cycles and having access to said subgroups, a master switch, a stepping magnet for each of saidswitches, an operating 10 circuit for said magnets effective upon each closure thereof to advance said master switch and said finder switch one step, means elective upon each advance of said finder switch off normal to advance it through one complete cycle, and means 15 controlled by said master switch in its successive positions for rendering said finder switch effective to test a different subgroup of said circuits during each operating cycle thereof.

12. In a nder circuit for testing any one of 20 a plurality of circuits which are arranged in main groups each divided into a plurality of subgroups, a plurality of nder switches each individual to a main group of said circuits each operable through a plurality of cycles and having access to a different one of said subgroups during each of its operating cycles, a master switch, a stepping magnet for each of said switches, means for repeatedly establishing an operating circuit through the magnet of said master switch and in rotation through the magnet of each 0f said nder switches whereby, upon each closure thereof, said master switch and one of said finder switches are each advanced one step, means effective upon each advance of a nder switch off normal to advance it through one complete cycle, means effective after each finder switch has completed a predetermined number of cycles to extend said operating circuit to the magnet of the next succeeding finder switch, and means controlled by said master switch in its successive positions for rendering said finder switches effective to test different subgroups of said circuits during successive operating cycles thereof.

JOHN W. TAYLOR. 

